This invention relates to improved supported chromium oxide polymerization catalysts and to methods for preparing and using them.
It is known that olefin polymers having excellent physical and chemical properties can be prepared using supported chromium oxide catalysts. Polymerization can be carried out in hydrocarbon solution or in a slurry process, the latter in which polymer is produced in the form of small particles of solid material suspended in a diluent. The simplicity of the slurry process makes it preferable in many instances. In such polymerization processes, the melt index of the polymer can be controlled by varying polymerization temperature--higher reaction temperatures generally increase melt index. However, this expedient is limited in slurry polymerization to about 110.degree. C. in a diluent such as isobutane, as a higher temperature causes the polymer to go into solution and results in fouling of the reactor from polymer deposition.
Other techniques are known in the art to modify the catalyst so as to produce a higher melt flow polymer. These techniques can also lower the shear response and narrow the molecular weight distribution of the polymer, but not to the extent desired for certain applications such as high speed extrusion or injection molding.
It is therefore an object of this invention to provide a supported chromium oxide polymerization catalyst.
It is a further object to provide a chromium oxide polymerization catalyst having lowered shear response for a given melt index.
It is a further object to provide a slurry polymerization process in which a polymer having excellent physical and chemical properties can be provided.